texts: Plausible reason for the Hot Neptune gap

texts:
This NASA article talks about how there's a dearth of hot neptune exoplanets that have been discovered so far.

https://www.nasa.gov/feature/goddard/2018/in-search-of-missing-worlds-hubble-finds-a-fast-evaporating-exoplanet

The article makes no attempt at postulating why this gap may exist. One possible explanation I have is that the silicate material of a terrestrial planet is difficult to vapourise and the surface gravity of a hot jupiter is too great to drive off hydrogen to any great extent, whereas a Hot Neptune lacks both the solid surface and the gravity to maintain its size.


texts: Are stars really "burning balls of gas"?

texts:
Even the NASA website refers to stars as "big ball of gas". It's a common statement that's popularized by movies and popular culture. However, stars are technically not in a gaseous state - they're in a state of plasma.

So do stars qualify as being made of "gas" as the proverb would suggest, or is that considered a misconception?


texts:
Forgive me if my question has an obvious answer, but I need to know the answer. I always thought that more massive/energetic objects had a stronger force of gravity than less massive ones; that is, the Sun would create a stronger gravitational pull than the Earth does.

Upon looking into Newton's law of universal gravitation, I began to wonder whether I misunderstood this. Does the gravity between the Sun and Earth have an equal force on both? As in, does the Earth pull the Sun with as much force as the Sun does on Earth? If so, would the Sun just resist accelerating to Earth because it is extremely massive? Mass is just an object's resistance toward accelerating from a force, right?

If this is the case, would it apply to GR? Sorry for hypothesizing, I'm honestly trying to get a grasp on it.


texts: Is albedo ever greater than unity? How is it calculated for various surface diffusivities and specular reflection?

texts:
I'm trying to understand how albedo would be calculated for a body with well-characterized surface reflectance properties, so that it's absolute magnitude could be defined. One could argue that this is going backwards (one should measure apparent magnitude, correct for distances and phase angle to extract absolute magnitude, correct for radius to extract albedo) but consider if the body were not smooth and diffusely reflecting.

Imagine two identically sized spherical bodies; one diffuse or matte white and the other somehow covered in liquid mercury and exhibiting specular rather than diffuse reflectivity, with no wind or waves.

Would they both have albedo of unity?

If these two reflective bodies were very irregularly-shaped and had an extremely large, flattened (much larger radius of curvature) face, would its albedo be larger than unity at times?

Here I'm looking for how astronomers and planetary scientists use the term albedo and not just a mathematical hypothesis. There may be ways this is dealt with practically.


texts: How large part of the sky has Hubble observed?

texts: I'm curious for an estimate of how wide a field the Hubble space telescope has observed. Say, for example, how many times it has changed its pointing, and what fraction of that were revisiting the same field.


texts: How is it possible that Saturn's gravitational acceleration felt by Mimas is stronger than Mimas' own surface gravity?

texts:
The surface gravity on Mimas is $≈ 0.063\text{ m}/\text{s}^2$ and Saturn's gravitational acceleration at the distance of Mimas' orbit is:
$$\frac{{GM}}{{r}^2} = \frac{{6.674 \times 10^{-11} \times 568.34 \times 10^{24}}}{{(185.52 \times 10^{6})}^2} ≈  1.102 \text{ m}/\text{s}^2$$
How can this be? An object on Mimas' surface would be much more attracted to Saturn than it is to Mimas. Shouldn't Mimas itself get ripped apart or is my math wrong?


texts: Trans-Neptunian Dwarf Planet near Heliopause

texts:
Is there any reason to think a Trans-Neptunian dwarf planet, the size of Pluto, could not exist in a Keplerian orbit 120AU from the Sun?

If such an object existed, and came within 0.1AU of the heliopause, is there anything about the environment near the heliopause that would prevent a stable orbit.


texts: Spacetime curvature illustration accuracy

texts:
When a particle travels through spacetime, this is sometimes illustrated moving in flat grid with various depths. How accurate is this way of imagining gravity?

Can the gravity well be infinitely deep?

Gravity well http://blakemaybank.com/wp-content/uploads/2012/12/Gravity_well.gif

Looking at this illustration, it suggests that there is an amount of distance going down the well. Is movement down the well limited by the speed of light? Remember, I'm aware that it is an illustration, and the question is about the accuracy of the illustration.

Is all gravitational wells infinitely deep at the exact center? So that if a particle could be made small enough to fit within the center, it would become a black hole.


texts: Can information leave the observable Universe?

texts: From what I gather, the observable universe is expanding at the speed of light. In addition, information cannot travel faster than the speed of light. It seems as though these two facts would imply that information cannot leave the observable universe. However, if that is true, it would either mean that the universe started spread apart, or that the observable universe is the entire universe. So, which of these explanations is correct?


texts: Calculating obital velocity from radial velocity

texts: I'm not really a physicist so I don't know if it's possible to get the real orbital velocity of a star from its radial velocity. I'm working on a paper for school and I want to get the mean velocity of selected open clusters to graph the galaxy's rotational curve, but in the databases I only encounter radial velocities. How do I get the real orbital velocity from those parameters? Thanks!


texts: Motion of the sun as observed from mercury

texts: I recently found this animation which shows the motion of sun as observed from mercury. It seems as if the sun stops in between, retraces a bit and then continues towards the west. What is the reason for this strange motion? Is it because of variation in the distance between mercury and the sun? Or is it because a day on Mercury is longer than Mercury's year?


texts: If Alpha Centauri A's solar system exactly mirrored our own, what would we be able to detect?

texts:
Suppose there was an exact replica of our solar system 4.4 ly away (people included). What would we be able to detect and with what telescope(s)? Which planets? Could we detect radio transmissions and/or any atmospheres?

I assume detection would be optimal if we were co-planar with the other star's ecliptic, so what would we see in the best- and worst-case (90°-view?) scenarios?

Post Script: a few months later I asked something like this during a von Karman lecture by Neil Turner.


texts: How does the center of a black hole can contain so much mass?

texts:
We know that

  "In the center of a black hole is a gravitational singularity, a
  one-dimensional point which contains a huge mass in an infinitely
  small space, where density and gravity become infinite and space-time
  curves infinitely, and where the laws of physics as we know them cease
  to operate."

So where does this mass go? A measure of how much matter is there in a object is called mass right? so how does that matter stays in a 1-D space?

We know that matter occupy space so how?

Edit: My question is about how can a singularity exists not about singularity itself.


texts: Were there any images of a transitary event of Jupiter or Saturn as imaged by a deep probe mission

texts:
I'm quite certain that there isn't any actual images of a transitory event of Jupiter or Saturn across the solar disk from say e.g.,s the Voyager probe, Pioneer 10/11 or even recent New Horizons mission?

Another related question I have, what would the approximate transit of Jupiter look like as seen from Saturn. The distance between Saturn and Jupiter is approximately 4-5AU, and the diameter of Jupiter is one tenth that of the Sun's. Are we looking at perhaps a transit which covers 10-15% of the solar disk?

N.B. Please note I understand that to observe a transit of Jupiter we have to be superior to it in the Solar System, hence I am asking for images taken by deep Solar System probes only.


texts:
I study Computer Science and I need to understand perfectly how is the process of publishing data to the virtual observatory, since I will start a project that seeks to develop another platform to enter data into the virtual observatory (in a more intuitive way if it's possible)

If I understand correctly I believe that this is currently done by means of http://cdsweb.u-strasbg.fr/publication-support#yourDataInVizieR and trough other web pages like http://www.cadc-ccda.hia-iha.nrc-cnrc.gc.ca/en/ or programs like http://aladin.u-strasbg.fr/

Does anyone understand the topic or where can I find documentation?

I feel a bit overwhelmed because in these web pages I see there are many concepts that I do not understand well catalogs, VOTable, I do not even understand very well what a virtual observatory, I do not want to ask my "thesis" adviser every 5 minutes about basic concepts.

Any help will be well received.


texts: We have a Celestron 70AZ 70mm f/10 refractor that has been working perfectly. Our children knocked it over the other day, and now when we try to view the moon, it fills the lens as a giant, blurry ball of light. Have they knocked something loose? We tried adjusting the different focus knobs but I am worried it's messed up and I have zero clue how to fix this, diagnose it or even where to go to get it looked at.


texts:
The semi-major axis a here,

what exactly does it represent? Point it out in the image below.


texts:
currently I'm working on astronomy problem which deals with finding redshift of a galaxy. But the first part asks to derermine apparent magnitude of a galaxy:

  Find what should be the minimum apparent magnitude of a spiral galaxy, whose distance could be calculated with sufficient accuracy from Hubble-Lemaître law and it's spectrum observations.

This apparent magnitude should be used later on together with given absolute magnitude (in visual band) to find the distance to the galaxy and in turn it's velocity from Hubble law.

I know how to solve the rest of the problem, but I cannot find the answer to this question. It seems to be theoretical question - with no calculations involved. I tried to think about it in to diretions:

There's some limiting brightness below which spectroscopy becomes impossible. But after searching it seems as long as there's light, it's spectrum can be analyzed. So the answer could be minimu magnitude observable from our telescopes? It seems to be about 30 but I doubt it's the answer because it's too much dependent on sources you use.
There's some limiting distance after which Hubble's law no longer holds. This direction leads to very difficult topics of cosmology, relating to general relativity and geometry of universe. My problem should be solvable even without special relativity. But as I understand, as long as you know the geometry of the universe, Hubble law holds. O have found that Hubble law works up to about z=1, after that you need know geometry and other advanced cosmological stuff. But again, numbers vary, so no defininte answer here.

Also it's worth noting that I probably need to find visual magnitude (because visual absolute magnitude is given). I assume it could redshift to invisible part of spectrum - maybe that has something to do with the answer.

Any help would be appreciated.


texts: How to draw an orbit from Orbital Elements using Transform3DGroup

texts:
I am trying to draw an orbit using the Transform3DGroup class in the C# .NET framework. I have orbital elements a, e, i, RAAN, ArgPer, trueAnom. The algorithm that spits these out for me also gives me x y z coordinates in ECI. For some reason, when I try to use Transform3DGroup to do my rotations in order to basically draw out where the satellite is supposed to go in ECI (using the orbit elements I gave above) my result is not matching up with the ECI coordinates that the original algorithm is spitting out.

That original algorithm is my truth data. But I don't want to use those coordinates, I want to draw the orbit myself with the Keplarian elements using Transform3DGroup.

Here is my code:

public Point3D calculatePosition(double trueAnomaly)
{
    double angle = trueAnomaly;
    double a = semiMajorAxis;
    double e = eccentricity;
    double b = (a * (1 - e * e)) / (1 + e * Math.Cos(angle));
    double SA = Math.Sin(angle);
    double X = b * CA;//(CA * CR + SA * CI * SR);
    double Y = b * SA; //(CA * SR + SA * CI);
    double Z = 0; //b * SA * SI;
    Point3D p = new Point3D(X, Y, Z);//new Point3D(Math.Cos(angle) * b,
    Math.Sin(angle) * b, 0);
    return p;
}
public void orient2OrbitalElements(double _time)
{

    orbitNorm = new Vector3D(0, 0, 1);
    positionPoint = calculatePosition(trueAnomaly);//Position of Orbiter
    Transform3DGroup tg = new Transform3DGroup();
    tg.Children.Add(new TranslateTransform3D(positionVec));//Translates to Orbit Position (relative to Parent)
    tg.Children.Add(new RotateTransform3D(new AxisAngleRotation3D(new Vector3D(1, 0, 0), inclinationDeg)));//Rotates Orbit to the set Inclination
    tg.Children.Add(new RotateTransform3D(new AxisAngleRotation3D(new Vector3D(0, 0, 1), RAAN)));//Rotates Orbiter's model RAAN
    orbitNorm = tgNoTranslation(tg).Transform(orbitNorm);
    tg.Children.Add(new RotateTransform3D(new AxisAngleRotation3D(orbitNorm, argOfPerigee)));//Rotates Orbiter's model to the Arg of Perigee
    Transform = tg;
    if (HasEphem)
    {
        Console.WriteLine(ObjectName + ": " + Transform.Value.OffsetX + ", " + Transform.Value.OffsetY + ", " + Transform.Value.OffsetZ);
        Console.WriteLine(ObjectName + ": " + cartesElems.xPos[ephemIndex] + ", " + cartesElems.yPos[ephemIndex] + ", " + cartesElems.zPos[ephemIndex]);
    }
}
public Transform3DGroup tgNoTranslation(Transform3DGroup tgWithTranslations)
{
    Transform3DGroup tempTg = tgWithTranslations.Clone();
    tempTg.Children.RemoveAt(0);
    return tempTg;
}

Basically I calculate an x,y position on the orbital plane w.r.t the Earth position using the basic eccentricity, semi-major axis, true anomaly formula as you'll see. Then I put the resulting coordinates into my Transform3DGroup as a TranslateTransform3D. This is the first object I put in.

Then I perform the rotations:
 --- a rotation is added for the inclination, rotating the position Vec about
     the unit x axis (1,0,0)
 --- a rotation for the RAAN, about the unit z-axis(0,0,1)
 --- then I create the orbit normal from that and perform a argPer rotation
     about that new axis

My understanding of how Transform3DGroup works is that it will apply the first one, then the second, and so on. Which means that the positionVec (which is just x, y coordinates in the reference frame of the 2d orbital plane) gets performed, then the rotations will all be performed as a rotation matrix on was is left after what was done before it.

Am I performing the argPer rotation incorrectly? RAAN or inc? Perhaps there is a more exact way to calculate my position in my orbital plane or a particular order of operations I must adhere to?

The more I think about it, the more I keep arriving at the thought that everything seems to be very simple if I use the Transform3DGroup and that I should be doing it right at this point, yet the ECI results don't line up.

But lo and behold, the Transform.Value.OffsetX/Y/Z does want to come close to agreeing with the truth data. Perhaps Transform.Value.Offset is not the X,Y,Z as I am thinking of it?


texts: Information about known objects in the Universe

texts:
I want to retrieve coordinates, photos, and information (size, atmosphere composition, distance from Earth, etc.) about every known object in the universe. Ideally, the information is available via an Application Programming Interface (API) for easy retrieval via a computer program.

What is the best free source of this information? Thank you for your help.


texts: What is the double peak in this case, please? What does the third mean? Many thanks


texts: Does this black hole magnetohydrodynamics equation even superficially make sense?

texts:
My question is about the journal paper mentioned in an Academia Stack Exchange post. Please understand that this paper has never been posted on arXiv, and I can provide only a link whose content is behind a paywall.

Summary of my question: it boils down to "whether a spatial coordinate of a fiducial observer can have a nonzero partial derivative with respect to the coordinate time."

I am interested in the validity of the central result of this paper. It is Eq. 4.24, which reads
\begin{equation}
\begin{split}
&\nabla\cdot\left[\frac{\alpha}{\varpi^{2}}\left\{1-\left(\frac{\omega-\Omega^{F}}{\alpha}\varpi\right)^{2}\right\}\nabla\Psi\right]
- \frac{\omega-\Omega_{F}}{\alpha}\nabla\Omega^{F}\cdot\nabla\Psi\\
&+ \frac{4\pi\dot{\varpi}}{\alpha^{2}\varpi}\left(1-\frac{\dot{\Phi}}{4\pi}\right)\frac{\partial \Omega^{F}}{\partial z} + 4\pi \frac{\partial}{\partial z}\left[\frac{\dot{\varpi}}{\alpha\varpi}\frac{\omega-\Omega^{F}}{\alpha}\left(1-\frac{\dot{\Phi}}{4\pi}\right)\right]\\
&+\frac{1}{\alpha\varpi^{2}}\left[\left(\frac{\dot{\alpha}}{\alpha}+\frac{\dot{\varpi}}{\varpi}\right)\dot{\Psi}-\ddot{\Psi}\right] + \frac{\dot{\varphi}}{\varpi}\frac{\omega-\Omega^{F}}{\alpha}\frac{\partial\Psi}{\partial\varpi}\\
&-\frac{16\pi^{2}\xi}{\varpi^{2}}\left(1-\frac{\dot{\Phi}}{4\pi}\right) = 0,
\end{split}
\end{equation}
where a dot on top of a symbol denotes a partial derivative with respect to the coordinate time $t$.

The above partial differential equation is supposed to describe the magnetosphere of a Kerr black hole. The authors use the spherical coordinates $(r,\theta,\varphi)$ and define $\varpi$ as follows:
\begin{equation}
\varpi \equiv \frac{\Sigma}{\rho}\sin\theta,
\end{equation}
where
\begin{equation}
\rho^{2} \equiv r^{2} + a^{2}\cos^{2}\theta,
\end{equation}
\begin{equation}
\Sigma^{2} \equiv (r^{2}+a^{2})^{2}- a^{2}\Delta\sin^{2}\theta,
\end{equation}
and
\begin{equation}
\Delta \equiv r^{2} + a^{2} - 2Mr.
\end{equation}
Note also that $\alpha$ is the lapse function defined as
\begin{equation}
\alpha\equiv \frac{\rho}{\Sigma}\sqrt{\Delta}.
\end{equation}

The functions $\Psi(t,\textbf{r})$ and $\Phi(t,\textbf{r})$ denote the magnetic and electric fluxes through an $\textbf{m}$-loop passing through $\textbf{r}$, where $\textbf{m} \equiv \varpi\hat{e}_{\varphi}$ is the Killing vector associated with axisymmetry.

What confuses me is the following: $\varpi$, $\varphi$, and $\alpha$ are simply spatial coordinates or combinations thereof, and their (partial) time derivatives should all be identically equal to zero because space and time coordinates are independent variables. This would render many parts of Eq. 4.24 nothing but convoluted ways to express the number zero.

I have also tried to follow the derivation of Eq. 4.24, and figured out that the authors implicitly assumed the following relations:
\begin{equation}
\dot{\Phi} = \dot{\varpi}\frac{\partial\Phi}{\partial \varpi}
\end{equation}
and
\begin{equation}
\dot{\Psi} = \dot{\varpi}\frac{\partial\Psi}{\partial \varpi}.
\end{equation}
Recall that a dot means a partial derivative with respect to time. As $\dot{\varpi}$ is identically zero, the above relations seem to be wrong.

However, what makes me somewhat unsure about my conclusion is that this paper is published in The Astrophysical Journal, a renowned peer-reviewed journal in astrophysics. (I have little expertise in astrophysics.)

Could someone verify whether my suspicion is well founded or correct me where I am wrong? Thanks in advance!


texts: I recently got my first telescope, a Bushnell reflector telescope. I spent a good while setting up my alignment. And I can see the moon perfectly! I went to look at another, Mars. I got it aligned in my finderscope. And i go to focus to see. And i see....basically the mirror in my scope with the lines. Its clearly getting light from the star....but i can't get it to focus on the star at all! Help!!


texts: When will the last total solar eclipse occur?

texts: Because the moon distance from earth grows, how long will it take, that it is no longer possible to see a total eclipse but only circular eclipses? And does it make a difference if you are on the mount everest or on sea level?


texts: What is the age of the universe if you were standing in a galaxy far far away such as GN-Z11?

texts:
If we agree with the universe starting from a Big Bang, 13.8 billion years ago, expanding at different rates governed by the Hubble constant of 67.4 km/s/Mpec, somewhere about 13.4 billion light years away a galaxy called GN-Z11 should be traveling at near the speed of light .924c and has been for about 13.8 billion years, relative to us. Applying relativity time dilation, the age of that galaxy, should, concurrent to us, be quite young, maybe 4 billion years, since its always (since the big bang) been traveling at relativistic speeds. So, in GN-Z11, is the time, since the big bang, and current state of evolution of the universe, truly much less than the time here?
More significantly is its future when its 13.8 billion, after 9.8 billion years, still unknown? I think it would be unknown because I truly think only 4 billion years could have passed there at the maximum, but the principle "that the universe has no center" tends to imply that it could be 13.8 billion years old there, and everywhere in the universe, like it is here. Which is it? Is it a fair question?


texts: What are the dimensions of LIGO Detector evacuated chambers?

texts: The LIGO complex is very large, but I assume that there is a very small chamber that actually does the science since they are measuring very small sheers. How big around is the actual evacuated chamber?


texts: is pan-starrs' data available to public users?

texts:
Anybody knows that?

The data products are all images or star catalogs?

This is the link:
http://pan-starrs.ifa.hawaii.edu/public/


texts: How to differentiate between images of a gravitationally lensed object

texts: The light coming from a distant galaxy or star can be distorted en route to Earth; this distortion is caused by the curvature of space when a massive object is present. My question is, how can we tell whether these doppelgangers are from the same galaxy (especially in the case of a star)?


texts: How could visible light be in pitch-black? Can it be?


texts: Why do medium stars collapse to form supernovae while big stars collapse to form black holes?

texts:
I understand that a star runs out of fuel slowly and gradually by fusing heavier and heavier particles (because of pressure by gravity) until the last heaviest particle is not able to fuse at the core of the star. This is where gravity takes over, and mass collapses (emitting gases out) to become a smaller dense star (with no energy left to give, just the gravity to absorb).

What I don't understand is why medium stars collapse to become a supernova which emits light while big stars collapse to form black holes which absorb all light.


texts: What is "g-98" in plots of stellar spectra?

texts: I was looking to find some example carbon star spectra and came across this page on the Harvard website which shows several plots of carbon star spectra. The horizontal axis is labelled as wavelength in ångströms, but the vertical axis appears to be labelled with "g-98" which I am not familiar with - any idea what quantity this is?


texts:
I found that Earth's perihelion occurs roughly two weeks after winter solstice similar thing happens for summer solstice and aphelion. I understand what those terms mean but I don't understand the reason for the two-week gap. I actually thought they occur on the same day.

What am I not understanding here? Can anyone explain the reason for the two weeks?

My understanding is that the earth's aphelion and perihelion occur on the major axis of earth's elliptical orbit. Why are they not the extremities of analemma which correspond to solstices? Where is my thinking wrong?


texts: What is the surface feature of the mesas in the Noctis Labyrinthus on Mars?

texts:
Caveat-my knowledge of geology is limited.

Is it known what is the surface like on the larger mesas in the Noctis Labyrinthus?  I trust the very smooth looking surface on this mesa is a product of low image resolution and not proof it is perfectly smooth.

This smaller mesa

is described as clusters of boulders and sand dunes on its surface. Do the larger mesas have these features?  Something like this,

Here are different surfaces from images taken by the Mars rovers. Is it known if the larger mesas have surfaces like any of these?

No dunes or clusters of large boulders.

Granted this is not entirely flat, but in the foreground it looks to my laymen's eye as surface bedrock with little sand.

Or generally flat rocks broken up and separate, with sand in between the rocks?


texts:
I noticed that last June the Sun was on the tropic of Cancer and moon on Madagascar:

Can you explain how that works and if they can get even farther in latitude?


texts: I was wondering if it would be possible to record the emission/absorption spectra of stars without expensive spectroscopy equipment. Would it be possible to somehow utilize diffraction grating? I own a Newtonian with a 130mm aperture if that helps.


texts: I am thinking of becoming an astronomer becuase I love looking through my telescope at night, and may want to make it my profession. Is there money to be made working at an observatory, or is the BLS median annual wage of $114,590 primarily from other career paths within astronomy? How hard is it to get a job at an observatory?


texts: Can I see Jupiter, Mars and Saturn through the National Geographic 76/350 Dobsonian Telescope?

texts:
Is it possible to see Mars, Jupiter and Saturn through the National Geographic 76/350 Dobsonian Telescope? It's the most affordable telescope I can get at the moment. Also, what about the moon? Will it look small?
Specifications

Diameter: 76mm
Focal length: 350mm
Magnification: 18x-117x
Eyepieces: 6mm, 20mm
Barlow lens: 2x
Compass
Moon filter


texts:
I would have assumed that the brightest radio source in the Universe is a quasar of some kind (perhaps 3C 273) given that the average pulsar has luminosity of $~10^{40} \text{Watts}$ and this is the brightest (optically) and is radio loud.

My professor seems to be of the opinion that the Arecibo message is louder, but that does not seem to pass the straight face test, given that it was only transmitted at a power of only $1\text{MW}$, and only a very small fraction of the pulsars output (a mere $1/10^{34}$th) would be sufficient to outshine the message.

Is one of these the brightest radio source in the universe (excluding the Big Bang if it matters), or is it something else entirely?

Additionally, is Watts even the right unit to be measuring this in?

Update: Here is the text of the question (it was a true/false, and the answer is true):

In 1975, Frank Drake and Carl Sagan sent a message in to the Universe, directed to extraterrestrials, from the radio telescope at Arecibo in Puerto Rico that, for a brief time, made the Earth the brightest radio object in the Universe.


texts: What a grating on GAIA RVS spectroscope?

texts:
What size of the grating on GAIA RVS spectroscope ?

How many lines on mm, or what defraction angle for 850 nm wavelenth light ?

I find some more details here: http://sci.esa.int/gaia/44092-gaia-rvs-grating/ and http://sci.esa.int/gaia/44093-state-of-the-art-grating-for-gaia/
The GAIA RVS grating has dimensions of 20.5 cm × 15.5 cm and 302.11 grooves per mm.


texts: How was Sugar Grove Station supposed to work?

texts: Sugar Grove Station was an outpost designed to listen in on Soviet radio signals reflected off the moon.  How would this work?  Wouldn't the radio signals be distorted by the imperfect surface of the moon?  And wouldn't they also be extremely weak since they wouldn't be beamed directly at the moon?


texts: Is it likely to ever discover three planets of similar mass revolving around a common barycenter, or would such a system be too unstable? Such triple planet system would look like a triangle once in a time.


texts: How many celestial bodies can I see with a 90x telescope?

texts:
I am a 9th class student so I'm a beginner at using a telescope, hence please help me.

How many bodies can I see using a 90x telescope?
How will I know when and where I should point a telescope to see planets?


texts: Time according to the gravity of Sagittarius A*?

texts: This may be a really dumb question (I'm more of a Biologist than an Astronomer) so I apologize in advance for my little knowledge pertaining to Astronomy, but, if I'm not mistaken, time is effected by gravity, right? So what is Sagittarius A*'s time compared to ours since it has so much stronger gravity? Do we know specifically the difference?


texts: nasaworking on a way to travel faster than speed of light

texts: even if they did how will time pass in that ship. Also that ship would require a infinite amount of energy to fight the drag.but what would provide the drag since in space there is nothing to resist?


texts: Are Saros series separated by "40" in numbering related in any way?

texts:
Can anyone help with this question? My question arose from an eclipse related observation that I made while looking into lunar Saros series 129. It is the Saros series number for the lunar eclipse that occurred on this past July 27, 2018.

First I saw that this Saros 129 series was "born" June 10, 1351 and the series will end June 24, 2613 (70 saros cycles of 18 years = roughly 1260 years). So, I became curious to see if another Saros series had ended just before Saros 129 began. I saw that Saros 89 (less by 40 in series numbering) had ended just four years earlier on July 23, 1347. Investigating further, I learned that there are roughly 40 different Saros series that are "active" at any one given time. So then I looked back further to lunar Saros 49 and observed that there does indeed seem to be a pattern involving roughly "40" less in the numbering (i.e. yes indeed, series 49 also ended just as series 89 began). These observations do seem to me as more than mere coincidences.

So, my question now is - Are these particular lunar Saros series (separated by "40" in numbering) related in any way? - series 49, 89, and 129. Is there another cycle or a "greater" cycle also at work here that I am unaware of?

Please see the comments below if what I am asking seems unclear - my comments might rephrase the question for clarity.

  Edit - I checked several other Saros series numbers separated by 40. I am adding additional information below - hoping that it might help find an answer to this question about relatedness of the "40
  apart" series.

Checking other series spaced by 40 numbering, these Saros series (that are numbered 40 apart – i.e. 40, 80, 120 etc.) always contain eclipses that are separated by ~1418 days between eclipses in each respective series (i.e. exactly 4 lunar years/48 lunar months).

Small Saros series (those containing fewer cycles - i.e. 70-74 saros cycles in the series) contain saros cycles that overlap minimally (Saros 129 which is only 70 eclipse cycles in length has no overlap and began just after Saros 89 came to an end 1418 days earlier). Note - zero eclipse cycles between them overlap.

Large Saros series (those containing more cycles – i.e. 80-84 saros cycles in the series) contain saros cycles that overlap accordingly (Saros 120 which is 83 cycles long thus began 13 cycles before Saros 80 came to an end with the overlapping eclipses each gapped by 1418 days). Note - 13 eclipse cycles overlap accordingly - again about 70 eclipse cycles with no overlap between these series.

It seems like about 70 eclipse cycles (constituting ~1262 solar years or ~1300 lunar years) is the standard length of each adjacent Saros series – considering the non-overlapping portion only – each non-overlapping portion separated by 4 lunar years.

  Edit - Thanks to anyone who can shed any additional light on this.


texts: Why do brighter stars/planets appear physically larger?

texts:
Look at Sirius and then some 2nd magnitude star. Sirius appears physically larger despite both stars subtending approximately the same angular diameter.
Callisto is far enough from Jupiter at its greatest elongation to be seen with the naked-eye, but a common reason it still isn't visible is because Jupiter's glare hides it.
To be clear, I'm not talking about Jupiter's additional reflected light making it harder to spot dim Callisto the same way the Moon makes nearby stars harder to see. Jupiter literally appears larger in the sky because of how bright it is. Why is that?


texts: Why does the Earth not have rings like Saturn or Uranus?


texts: After a radio image has been cleaned it is advised that the fluxes be rescaled if the source is near the galactic plane. Can someone explain why this is needed?


texts: What it would look like to observe people with a different time flows?

texts:
As I learned, that the bigger gravity source you are influenced by the more slow time ticks for you, the farther away you are from a gravity source the faster times ticks.

So Imagine two different planets where on each people live, one planet is massively big (time ticks slow), the other one very small compared (time ticks faster). How would it look like for bigger planet people to observe smaller planet and for smaller planet people to observe them. Hope it makes some sense.


texts: If two white dwarfs collided, would they become a star?

texts: Would the temperatures during such a collision be able to ignite nuclear fusion, bringing the dead star back to life? If so, would it only be able to fuse for a short time before running out of fuel, or would it turn into a full-fledged star?


texts:
It is often possible to find a list of the materials that compose the cores and the mantle of plantes (often they are iron and/or nickel).

How can these materials be determined in our Solar system? In other words, for example, how can we know that Jupiter's core certanly contains iron?


texts:
Could there be another planet between Mercury and the Sun?
Kepler found a lot of planets orbiting its star even 20 times closer than Mercury.
If there were such planets in our system - would it be possible to discover them?


texts: Understanding Scatter in Data collected for HR Plot

texts:
I was recently asked a question for a lab I am doing for an astronomy class where we collected data from the M44 cluster (through Google Sky) and used information about the Hyades cluster to estimate the age and distance of M44. Anyways, I was asked whether there was more scatter in my M44 data or the provided Hyades data. There was definitely more scatter (see below) and I'm not sure how to explain it. I believe it is because of these reasons:

There is measurement error.
And I might be picking up background/foreground stars This would definitely lead to scatter. If I measure a foreground star and assume it is as far as the M44 star cluster, it would appear brighter than if it were truly in the M44 cluster.

Am I on the right track or am I missing something? Any help would be great. Here is my data, the green is the collected M44 data.


texts: Ionization Fraction of Helium in alternate, He dominated early universe

texts:
I'm asked to verify an expression for the ionization fraction of helium in a universe in which helium dominates baryonic matter.

I'm given that the ionization fraction $X = \frac{n_{He^+}}{n_{He^+}+n_{He}}$ with a given ionization energy $Q_{He}$, and number densities by the formula $n_i=g_i(\frac{m_ik_BT}{2\pi \hbar})^{3/2}\exp(-\frac{m_ic^2}{k_BT})$. Also the baryon-photon ratio in this case is $\eta=\frac{4(n_{He^+}+n_{He})}{n_\gamma}$.

I'm trying to show that by putting all this together I get something with the form:

$\ln(\frac{1-X}{X^2})=A+\ln(\eta)-B\,\ln(\frac{Q_{He}}{k_BT})+\frac{Q_{He}}{k_BT}$. where $A$ and $B$ are constants.

When I plug everything in, I got $\frac{1-X}{X^2}=\frac{n_\gamma}{8}\eta \, \exp(-\frac{Q_{He}}{K_BT})$, which gives:

$\ln(\frac{1-X}{X^2})=\ln(\frac{n_\gamma}{8})+\ln(\eta)+\frac{Q_{He}}{k_BT}$

Any ideas of where I might have gone wrong?


texts:
We know that we can use Doppler effect to determine equatorial velocity of a planet. We send a electromagnetic wave with wavelength $\lambda_0$ to an equatorial edge of planet and we get wavelength $\lambda$. So we can use Doppler Effect formula, (assuming $v \ll c$)

$$\frac{\Delta\lambda}{\lambda_0} = \frac{v_r}{c}$$

such that $v_r$ is radial velocity. But $v_r$ = $2*v_p$. And $v_p$ is equatorial velocity of planet.

I want to know where does the $``2"$ comes from. Someone said it is because of receiving the signal and then reflecting it. I want to know the math behind this not just to say receiving and reflecting. Why do we need to multiply the velocity by 2 to get the radial velocity?


texts: Which planets have abundant amounts of photoluminiscent matter? Photoluminiscent or chemoluminiscent whatever the glow-in-the-dark stuff it may be. Planets or bodies which have good enough the photoluminiscence(light absorb and then release) property.


texts: Effects of a rogue planet passing between the Earth and the Moon?

texts:
The plot of Thundarr the Barbarian got me thinking about the science behind

  the passage of a runaway planet between the Earth and the Moon

and its effect on the Earth and Moon themselves, as well as their orbits.

For the following scenario:

planet has half the mass of Earth, and is moving at 16km/s
moving perpendicular to the plane of the ecliptic
passes directly between the Earth and the Moon with closest distance to Earth of 192,500 km

What sort of effects, if any, might this have on Earth and those of us residing there during the passage in later months and years?


texts: How do OGLE-III and GAIA measure the mass of free microlensing black holes?

texts:
What is the "hypothesized lower mass gap" between 2.5 and 5 solar masses? eventually links to Constraining the masses of microlensing black holes and the mass gap with Gaia DR2.
The angle of deflection of light passing a massive object is given by:
$$\theta = \frac{4GM}{r c^2}$$
where $r$ is the minimum distance from the mass that the light passes.
If two black holes pass by a line-of-sight to a distant object and their speeds and distances of closest approach $r$ both scale linearly with their mass, they produce the identical deflection magnitude and time dependence.
Question: How then can such an observation of a free-floating black hole be used to determine its mass? What additional information is necessary? I see from the article that parallax is involved, but without knowing the distance to the black hole I don't see how this is enough to work out a mass.


texts:
I am getting conflicting accounts everywhere. I heard astronomers saying Polaris is a 3-star system. Some say they are binary but are optical-doubled with a 3rd.

Phil Plait a renowned astronomer wrote in his book Bad Astronomy that Polaris is sextuple star system.

The above text reads

  ... but due to its proximity to the NCP it has taken on the popular name of Polaris. The star itself is actually rather interesting; it's really a multiple star consisting of at least six stars in orbit around each other.

While in his Crash Course Astronomy Series He said Polaris was a Pentuple star system.

Now, I am not saying Phil is doing anything wrong. I am actually a fan of him, enjoyed reading the books he authored. He may have said one or the other by mistake. I tried asking him this question, it does not look like there is an effective way to submit him an Errata or ask him questions like these.

So my question is, how many stars are Polaris and are they all gravitationally bound to each other or some of them are just Optically grouped?


texts: How far do we have to go to leave the galaxy?

texts:
How far is the sun from the edge of the galaxy in the following directions?

Moving along the shortest path parallel to the galactic plane (outward radially)
Moving along the longest path parallel to the galactic plane (radially presumably passing through the center)
Moving along the shortest path perpendicular to the galactic plane ("up")
Moving along the longest path perpendicular to the galactic plane ("down")

I realize these distances may be somewhat fuzzy (edge of galaxy is probably hard to define exactly, galaxy isn't exactly an ellipse, computing distances like this are hard even if the boundaries weren't fuzzy), so ranges and uncertainties are fine. I don't expect precision like computing whether certain points on the edge might be closer than just going straight out from the center and the like.


texts: On the consistency of different well-polished astronomy software

texts:
I have purchased a custom wedding band from a seller that claims the ring will show the constellations visible at the horizon on a specific date at a certain date and time.

However, I have fired up Stellarium and set it up to look at a specific example shown in the product page - and I cannot find any set of parameters which looks like it: Jupiter's not in the right place, some constellations are under the horizon (Virgo, Libra, Leo), some others very high in the sky (Andromeda, Perseus)... Although it's the first time I use Stellarium, I don't see how I could make a mistake:

Location set to London Bridge (even the preset London to be sure)
Date and time set just like in the example (double-checked 5 times)
Cylindrical projection (tried all of them though), offset -30%
FoV 210° (to have the 360° mapped to the entire screen - I have
absolutely no idea why 180° does not work)
Elevation lines horizontal (although I tried moving around to align everything like in the pictures for every projection method - in vain)

Here is the example:

Am I missing something, or is their sky map wrong? If yes, is it possible that two pieces of well-polished software like Stellarium and this one (looks just as good as Stellarium) have different star locations? How come (isn't there a standard star almanach?)?


texts:
Need Help. I have a problem with my 50 mm objective with 500mm focal length refractor telescope. I can see moon clearly. I have 18 mm and 9 mm eyepieces.

I see Jupiter and Venus as a bright point when focused at 30x or 60x and I see white disk when out of focus. While I have successfully resolved double stars with it and I saw Pleiades star cluster clearly with it, I cant see the moons of Jupiter.

Should I buy 3x Barlow to see the bigger image? is low magnification is my problem?
Or there is another solution to this problem.?


texts: Why shouldn't you look at the Sun through a camera using eclipse glasses?

texts:
One of the NASA approved eclipse glasses manufacturers has the following warning on one of their eclipse glasses:

  Do NOT look through a camera, telescope or binoculars with ANY eclipse shades on!

I can see how the enlarged image of the Sun through a telescope or binoculars might overwhelm the eclipse glasses' filters.

But how would looking at the Sun using a camera's viewfinder or LCD screen reduce the effectiveness of eclipse glasses (assuming you're not using any zoom function)?


texts: Fitting PSF to an image to get total flux (Python)

texts:
I have an image of a single unsaturated source (asteroid) that I'd like to fit guesses for various model PSFs to, so that I can then compare the total flux in each case. I've only recently begun studying PSFs, so I am a bit confused.

I know that astropy offers a package called photutils.psf which does the PSF fitting for you, but I would like to better understand what's going on behind the fitting, and how the flux of the source is resolved from it. Furthermore, when I tried using this package, I wasn't getting any outputs anyway (it said a source was found, but that it didn't meet the sharpness or roundness criteria, despite me changing those parameters around quite a bit). I have tried something simpler below (where model_data is the 2d numpy array representation of the PSF, and img is the asteroid image data):

import numpy as np
from astropy.modeling import fitting
from photutils import FittableImageModel

y, x = np.mgrid[:np.shape(img)[0], :np.shape(img)[1]]

fitter = fitting.LevMarLSQFitter()
model2d = FittableImageModel(model_data)
result = fitter(model2d, x, y, img)
print(result)
print(np.median(img), np.max(img), np.sum(img))

The output I get is then:

flux       x_0       y_0

-----     -----     -----

9282.784  466.44    531.69

2705.82 25301.56 980812434.98

However, these are not the correct centroid coordinates of the source in the image. The correct coordinates are around (300, 300). Also, to get this result, the PSF data was normalized so that the max intensity point is 1. But when I change this normalization so that the total flux (sum of all the pixels) is 1, the result for the flux after fitting I get is way higher.

I'm not sure what's happening behind the scenes here (I got lost in the documentation), and I'm not sure if I can trust any of these flux values, considering the x_0 and y_0 are always off. So I'd like to better understand -- how exactly does one find the flux of an object by fitting it to a PSF? And how should that PSF be normalized?


texts: How big is the universe. How does it works?

texts:
I watch this movie:

How big is the universe
https://www.youtube.com/watch?v=b34gwCKIM50#t=50m00s

I can't understand where is there triangle with 180 degrees.

What exactly is this red spot on the cosmic microwave background?

Is it a base of triangle? How does it appear that this spot is smooth surface but not a concavity?


texts: Given telescope diameter and focal length only, how can I find the size of a produced image?

texts: I'm new to astronomy and I'm struggling to understand this concept. For example, A 20-cm diameter telescope with a focal length f = 2 meters produces an image of the moon 1.7 cm in diameter. How large is the image of the moon produced by a 5-meter diameter telescope, focal length f = 16.7 meters?


texts: Is there a formula to calculate any star mass from Luminosity, Radius, and/or Temperature (K)?

texts:
I know there is the Mass-Luminosity relationship, but I am wondering if there is a more accurate formula I can use based on the data points I have generated:

Absolute Magnitude (based on class and type of star in relation to H.R. diagram)
Temperature (same as above; in Kelvins)
Luminosity (calculated from Absolute Magnitude)
Radius (calculated from Temperature and Luminosity)

Basically the last 2 pieces I need to calculate are Gravity and Mass which is a pain because the one formula I have requires one of them to find the other. So far everything is calculated from another piece and so all of my values are in line with each other.
I could use the Mass-Luminosity relationship if I must, but are there any other formulas? I couldn't find anything that didn't require gravity. I doubt it, but there isn't by chance something to calculate gravity from my data points is there?


texts: Future of CMB observations: How will our knowledge of the early universe change?

texts:
The Planck satellite has been presented and awaited for a long time as the ultimate experiments for measuring temperature fluctuations in the cosmic microwave background (CMB) over the full sky.

One of the big questions that still need answer and that Planck might help clarify is about the dynamics and driving mechanisms in the first phases of the universe, in particular in the period called inflation.

Thankfully there is room for improvements at small scales, i.e. small pieces of sky observed with extremely high resolution, and more importantly for experiments to measure the polarisation of CMB.
I know that for the next years a number of polarisation experiments, mostly from ground and balloons, are planned (I'm not sure about satellites).

For sure some of these result will rule out some of the possible inflationary scenarios, but to which level?

Will we ever be able to say: "inflation happened this way"?


texts: Formation of elements in the Sun other than helium

texts: I just learned that the Fraunhofer lines of the Sun's spectrum indicate that the Sun contains various elements other than just hydrogen and helium (for example, Na and Fe) but don't the Sun's p-p chain reactions only involve hydrogen and helium? So where do these other elements came from?


texts: Position of Neutron stars in H R diagram

texts: Why is that Neutron Stars are never depicted in an H R diagram? They can be placed in the bottom right corner but you will never find any diagram in literature showing neutron stars.


texts: Hello I'd like to ask what are steps to get a light curve wfrom Kepler/K2 database. Let's say I'd need to get light curves of 10 random K type stars. How should I access the databases and what should I do next??


texts: Would the solar corona be visible from the distance of the outer planets?

texts: If the photosphere was blocked, before the sun rises on an airless moon in the outer solar system, would either the inner or outer corona be visible to the naked eye?


texts: Could a heavy exoplanet squeeze water into being liquid and 4° warm?

texts:
Let's say there is an exoplanet orbiting its host star well outside its habitable zone. Suppose further that it has a lot of water, being perhaps comparable to earth when it comes to the volume ratio of water compared to the remainder of the planet.
Now water at its densest is 4° Celsius warm. My question is: Could the exoplanet's weight force the water into a liquid state by squeezing it to the temperature of lowest density?
My intuition would say no, because after all, temperature is nothing but molecular movement, and every kind of accelerated movement is supposed to lower the total energy of a system, which seems to be absurd.
Also, this seems to be an explanation of the so-called "faint young sun paradox", which is well-known in geology.
If the answer differs from that, I would be pleased to read an explanation of how my reasoning was flawed.


texts:
Based on reading the wiki page of the big bang, a simple question with a probably not a simple answer came to mind.

The statement on the wikipedia page was.

  Since Georges Lemaître first noted in 1927 that an expanding universe could be traced back in time to an originating single point, scientists have built on his idea of cosmic expansion.

My question is:  Have we measured or found evidence of a detectable origin point of the big bang?


texts: Variable and Multiple Stars in Hipparcos

texts: Is there any way I can tell from Hipparcos catalog which stars are Binary/Multiple Stars and which are Variable stars?


texts: Have I nearly found the event horizon of a black hole?

texts:
The calculator on the website https://planetcalc.com/1758/ cites Wikipedia's Gravitational acceleration and implements:
$$g = G \frac{M}{(R + h)^2}$$
For a black hole with mass $M$ equal to 5 solar masses with a diameter $R$ of 1 meter at a distance $h$ of 1500 km, the acceleration given is 296000 million m/s^2 which is nearly close to trap light.
So one might expect the event horizon to be at least close to that. Is this correct?


texts: Is it possible to calculate distance between two objects?

texts: For example: I want to know the distance between Alpha Orionis (Betelgeuse) and Beta Orionis (Rigel). Is this possible? You only know the distance from Earth. May be this is a math problem and not an Astronomy question.


texts: When light is red shifted from distant galaxies, the photons have lost energy. When dark energy pushes objects apart, those objects have gained energy from a larger gravitational potential. Is the amount of energy that dark energy applies to push objects apart equal to the amount of energy lost because light from distant galaxies is red shifted?


texts: CCD in telescopes: Observation and Astro-photography

texts:
This is a Celestron CPC 800 XLT telescope. It doesn't seem to have charge-coupled device (CCD) attached. Can a CCD be attached separately to this 8" Schmidt-Cassegrain telescope even if it didn't come along with the package?

For astro-phtography how cheaper and cost-effective would a DSLR camera be than CCD? I think it is better to suit yourself with a CCD than a DSLR camera for quality photography.

Do excuse this novice who is here to learn.


texts: Average number and type of planets orbiting given star types

texts:
I don't need "exact" numbers, but I'm trying to create an auto-generator for a stellar map to crate stars that are "somewhat" accurate.

I'm looking for numbers on the average number and type of planets around each type of Star, and whether they could be a good source for mining or are they good places for life?

So, like, I need an idea of how what is the average number of planets that are expected around a G class star, how many are expected to be Gas giants or terrestrial and what is the likelihood of them being in the habitable zone and containing life.

If I base it on Sol I would suspect about 10 planets, 50% terrestrial and 50% gas giants, with 1 to 3 in the habitable zone and a 30% chance of life developing. I'm looking for something similar to this type of information.

I assume this wouldn't be good for across the board or even for G class stars in general.

So I guess I'm looking for estimated planetary statistics based on spectral type? I don't want a list of exoplanet and stars that I have to look through and then make a bad guess at, but a good estimation of these numbers based on known facts.

This Question doesn't have a simple enough answer for me to grab onto to use effectively without just making too much stuff up anyways.

This Question seems to have pieces of information that I'd use to work something out, but it would likely by far more inaccurate than what other people would come up with and there might be better answer than that so I ask my question.


texts: Why doesn't Earth's leading tidal bulge cause the Moon to start spinning in the opposite direction than its original spin?

texts:
Please refer to the image below:

My question is, why doesn't Earth's leading tidal bulge (encircled in the green circle 1) pull on the moon's tidal bulge (encircled in green circle 2), leading to a force $F_{1,2}$, and consequently a torque on the Moon's tidal bulge, leading it to start spinning as shown in the image. This spinning direction would be opposite of the original direction of the Moon's spin, which got retarded and eventually stopped due to tidal interactions over billions of years.
Please note: I know that Earth's tidal bulge causes the Moon as a whole to increase its velocity which is causing the Moon to be drifted away from the Earth, but that is not what my question is.


texts: Proper Terminology For Satellite Going Into Planets Shadow

texts: What is the proper terminology for when a satellite or spaceship passes into a planet's shadow or night side? Is it crossing the terminator? Passing into eclipse? If there is a different name for coming back into the light, what would that be as well?


texts: Estimates for "undetectable" planets in extra-solar systems

texts:
As far as I've been able to determine, it appears there are some significant  limitations on our ability to detect exo-planets that are below certain sizes or beyond certain distances from their stars.

It seems that Uranus and Neptune are about as small as we can detect unless the planet is relatively close to its star, and Earth is near the lower size limit we can detect, regardless of proximity to the star.

In other words, our bias toward detecting large planets, and planets close to their stars, means I have been able to find reasonably good data regarding how many of those kinds of planets are generally present in a system.  But since we can't seem to tell (by direct observation) how many small planets are generally present in a system, or how many planets are present that are just too far from their star for us to detect, I'm struggling to find data regarding how many plants of those types are common in planetary systems.

For example: our system has 8 planets, 4 rocky, 2 gas giants, 2 ice giants.  But chances are that if we were observing (with current technology) our own Solar system from some other solar system, we'd probably only see 2 to 6 of them.  We'd almost certainly see Jupiter and Saturn, and we'd almost certainly NOT see Mercury or Mars (they're just too small), Venus and Earth "might be" close enough to the sun for us to see them even though they are on the small end of what we can detect, and Uranus and Neptune also "might be" detectable, even though they are small-ish for their distance from the sun.

My specific question:  How many planets are in a 'normal' or 'average' planetary system, according to humanity's best scientific theories available so far?(beyond what we can simply detect.)  In other words, where can I find reasonable scientific research data sets, models, estimates, theories, evidence, etc., that describe the likelihood of "undetectable" (or very hard to detect) planets being present in a planetary system?

This is my first question on Astronomy stackexchange, so please be gentle, but don't hesitate to provide constructive criticism if I'm doing anything wrong.


texts: Okay so scientists use the red shift to know that galaxies are moving away from us faster than light. How do we know that its not just red light from red stars. i mean most stars are in the infrared spectrum.


texts: What is a "semester series" for eclipse prediction? - Solar saros series 147 and 152

texts:
Yet again, I just stumbled across another baffling eclipse question (actually, two now). I was interested in two eclipses (Saros series 147 and 152) that will both be occurring in 2021. Each of these eclipses will occur relatively near the opposite poles of the earth. The Wikipedia link for saros 147 provided the following additional information:

  "Solar eclipses of 1964-1967[edit]

  This eclipse is a member of a semester series. An eclipse in a
  semester series of solar eclipses repeats approximately every 177 days
  and 4 hours (a semester) at alternating nodes of the Moon's orbit.[1]
  "

These are both fairly young saros series. Saros 152 is comprised of 70 eclipses, while saros 147 contains a total of 80 eclipses. Observations: They are 10 eclipses difference in length - They each began 180 years apart. So, following on from that, I checked out their ending dates and found, much to my amazement, that both series end during the exact same year - exactly 177 day apart - in the year 3049 !!!

So, since this additional information, my new questions concerning these eclipses have become:

  1) What exactly is a "semester series" for eclipses (any detail)? The day spacing definition is obvious but doesn't explain much else.

  2) Does this mean that some or all eclipses in series 147 and 152 are in
  the same semester series?


texts: How do we know degenerate matter exists on white dwarfs? Is it purely hypothetical or have we observed it before? Have we ever created a form of degenerate matter on earth?


texts: What effect does time dilation have on bodies orbiting close to Black Holes?

texts:
As far as my (somewhat basic) knowledge of astrophysics goes in general the closer to a star your orbit gets smaller (because you travel less distance) and faster (because you're deeper in the gravity well and need to go quicker to avoid falling in.)

But what happens on bodies orbiting close to black holes and subject to time dilation as far as an external observer is concerned?

This question isn't about interstellar but it's a good point of reference. Take the water planet they drop onto where gravitational time dilation is enough to make 20 or 30 years pass to the external observers while only a few hours pass for them. (I don't know if this is a realistic rate(?) of time dilation.)

What would the apparent orbit of their planet be to the external orbit and what would his orbit appear to be for them? Even though he is further from the star would he complete more orbits in the elapsed time despite being further from the centre of the system? And would they appear to be orbiting "slower" than him from his point of view.

Out of interest, though perhaps a second question is more appropriate, how would the apparent difference in relative orbital speed affect rendezvous manoeuvres if at all?

For example would the standard decelerate and fall into a lower orbit still apply but the trajectory plotted from a static observer look weird as time dilation set in? Or would the manoeuvre have to be different to account for the apparent velocity difference? What would a non-circular orbit look like in this system when observed from a circular orbit?

Also is the velocity required for orbit so close to a black hole high enough that speed based relativity also comes into play?


texts: Other than relative temperatures, is there any difference between a red sun and a yellow sun?

texts:
As time goes by, explanations for unusual things that exist in fiction get disproven. We know radiation probably isn't going to create a Giant ant that destroys everything, we're pretty sure we can't use ray guns, and the idea that falling into a black hole would put you face to face with Ming the Merciless is laughable.

Comic nerds might have guessed from the title why I'm asking, but I'm curious if there could still be any reason to latch on to to explain why superman is powered by a yellow sun, but not a red one? Does the higher temperature of a yellow sun create a higher level of some concentration of exotic particles, or once you get to the point of [solar fusion] is the output pretty much exactly the same?

Note: This question is not about superman, or his powers, just if there is any (temperature aside) meaningful difference between a yellow and red sun (that could be conveniently used by DC to still justify superman's powers).


texts: How does a neutron star emitting magnetic-dipole radiation lose angular momentum?

texts:
Is the ultimate reason that the neutron star loses angular momentum that the magnetic-dipole radiation is not spherically symmetric and photons have momentum?

Just for the loss of angular momentum, particles emitted from the neutron star surface are not necessary, right?


texts: What percentage of a spiral galaxy is the center/bulge?

texts: Is there a credible source that can tell me how big the bulge of a spiral galaxy is compared to the rest of the galaxy? Unfortunately, I could not find any.


texts: What specific parts of the Milky Way do we see when we look up?

texts: I know we can't see the center.  But do we see any of the central bulge?  Are we just seeing nearby stars in the Orion Spur where we are?  In the Sagittarius Arm?  In the other direction, are we seeing any of the arms further out from ours?  Where are the dust clouds we see, in relation to these arms?


texts: Why is the eccentricity of Triton so low?

texts: Triton is very special, since its orbit is retrograde respect Neptune, its obliquity is near 0° and its orbit is almost circular (according to Harris (1984) is $5 \times 10^{-4}$). I'm familiar with the theory that claims that Triton was several Myr ago a KBO that was captured by the gravity of Neptune, but I didn't find an actual explanation of why the orbit is almost circular. Can anybody give me some theories that actually explain the cause of this? I've heard about tidal circularization, but I don't know if that explains it at all...


texts: I am thinking about making a telescope. I have a 100cm focal length lens which I can use as objective lens in my telescope. So which is the best focal length I can use as eyepiece in my astronomical telescope?


texts: Where can I find historic rates of meteor impact events?

texts:
I have a friend that watches a lot of TV news and they have the impression that impact events are becoming more common than in recent memory. Does the evidence support or refute this?

Thinking it would answer the question, I've tried to find data on the annual rate of impact events. But maybe there is another way to look at it.


texts: How many times has the sun orbited the galaxy?

texts: Even if our solar system is not in a stable orbit around the center of the galaxy (is it?), since the birth of our sun, how far has it traveled relative to the center of the galaxy?


texts: What are realistic and unrealistic values for the high energy gamma ray spectral indices of pulsar wind nebulae?

texts:
This summer, I was working on a project fitting very high energy gamma ray spectra of the Crab Nebula, a pulsar wind nebula. At energies about $\sim$1 TeV, a simple power law suffices, i.e. $\phi(E)\propto E^{\alpha}$. Most power law models of the Crab find a spectral index of $\alpha\simeq-2.47$ (e.g. Hillas 1998, $\alpha=-2.49\pm0.06$). Using a normal analysis method on one dataset, I found $\alpha=-2.469\pm0.02248$, which matches the literature values quite well.

Using the modified analysis method I was testing, however, I found $\alpha=-2.316\pm0.02040$. We believe that method is a poor choice because I showed the calculated flux values were incompatible with previous observations. The spectral index also cast doubts on its viability. However, I was wondering if, for a random pulsar wind nebula, a slightly harder spectral index like $\alpha=-2.316$ is unphysical, or reasonable.

Therefore, I'm wondering: What is the distribution of spectral indices for power-law models of very high energy spectra of pulsar wind nebulae? Do we have a large enough population of pulsar wind nebulae that we can determine what a typical value for $\alpha$ is, and what range we would expect a spectral index to fall into? In other words, could if I had obtained a certain value for $\alpha$, what value would it need to have for me to reject the result as unphysical?


texts: Telescope optical tolerance from central axis

texts: I sometimes encounter telescopes/binoculars with a decent objective diameter but are very difficult to use because they only work when the eye is looking down the central axis (i.e. in the centre of the field) or close to it; if you try to focus on an object towards the edge of the field of view it goes black.  Is there a name for this tolerance?  It doesn't sound related to exit pupil but probably is something to do with eye relief - i.e. the distance of eye to the eyepiece lens.


texts: I assume expansion implies an increase in the volume of the universe. But if the volume of the universe is increasing, does that not imply the area it is expanding into's volume is being reduced?


texts: Why aren't there blue elliptical galaxies or yellow spiral galaxies?

texts:
Elliptical galaxies are universally old and yellow; about three-quarters of all ellipticals have no significant star-forming gas or dust left, and even the quarter or so of ellipticals that are still forming stars do so at much lower rates (and with gas and dust reservoirs that are far closer to depletion) than is the case in irregular and spiral galaxies.

In contrast, spirals and irregulars have large amounts of gas and dust, breed stars like rabbits, and, as a result, contain huge quantities of young, bright, blue stars.

Why are elliptical galaxies so universally old, yellow, and star-forming-material-starved, while other types of galaxies are young, blue, and rich in gas and dust?  Shouldn't there be a bunch of young, blue, gas/dust-rich ellipticals complementing their yellowed seniors, and many yellow, geriatric, no-longer-star-forming spirals and irregulars to go along with the young blue star factories?


texts: Is the difference in time between the sideral and synodyc month - constant?

texts:
According what I read, the sideral month isn't constant (here)1, as well as the synodic month that isn't constant (here)2. Now, as far as I understood, because the sideral month isn't constant therefore the synodic month isn't constant too.

My question is: Is the difference in time between the sideral and synodyc month (around 30 degrees) - constant, or it may be various too?

The information above was taken from here:

1) synodic month. (n.d.) The Great Soviet Encyclopedia, 3rd Edition. (1970-1979).

2) sidereal month. (n.d.) The Great Soviet Encyclopedia, 3rd Edition. (1970-1979).


texts: Why is the magnetic reconnection(of sun) effect many orders of magnitude faster than predicted by standard models?

texts: Why is the sun's magnetic reconnection effect many orders of magnitude faster than predicted by standard models?


texts:
I'm looking for a mathematical justification of the longitude of periapsis as it makes no mathematical sense to me as a notion. By definition, it is the sum of the argument of periapsis and the longitude of the ascending node, each of which makes sense on their own.  I understand these latter two notions geometrically. But I don't know what the longitude of periapsis is supposed to represent.  The longitude of the ascending node is an actual angle measured within the ecliptic, while the argument of periapsis is an actual angle measured within the given bodies orbital plane.  These two planes are skewed with respect to one another by an inclination angle.  Thus, you cannot mathematically just add the two angles together; it geometrically has no representation.  At least not without making certain unmentioned assumptions which cannot possibly be true for all bodies.  I cannot make sense of why it seems to be the preferred orbital parameter.

This question is not a duplicate of What is exactly the "longitude of the perigee". The two-sentence afterthought regarding this topic therein doesn't adequately respond to the spirit of my question. I'm looking for a mathematical justification for its use, including why its regarded as more accurate than splitting up the angles into their separate components.


texts: Trying to categorize moon phase using fraction illuminated

texts:
I have a dataset with events that are occurring over a period of 5 years. I am trying to determine whether these events are related to moon phase. I downloaded the "fraction of moon illuminated" for the time period, but am struggling to define the cutpoints for crescent, quarter, and gibbous. These are defined by the USNO as 0.25, 0.5, and 0.75 (http://aa.usno.navy.mil/data/docs/MoonFraction.php) but I am seeking to divide the period into 8 bins of equal time. I think it is a sine function, where the period is 29.53059 days. If I divide this by 8, I get: 3.69132375 days.

Can anyone help me determine the fraction illuminated (y) that corresponds to these outpoints?

0 days

3.69132375 days

7.3826475 days

11.07397125 days

14.765295  days

18.45661875  days

22.1479425  days

25.83926625  days

???

Thanks


texts: What is the reason the Chinese want to measure the <30Mhz radiation behind the moon?

texts:
On the Earth it is difficult to detect radiation below 30MHz from space due to our atmosphere. But why do they want to measure this radiation behind the moon? It has something to do with the big bang, but isn't the frequency of CMB not around 160GHz?

The Chinese are planning to launch a satellite with a 30 MHz antenna on it in 2018.

Aren't there already enough decent measurements of the CMB? So what would they like to find concerning the Big Bang?


texts: Are white dwarf stars supported by proton degeneracy as well?

texts:
In general, fermions form a degenerate gas under high density or extremely low temperature. It's clear that white dwarf stars are supported by electron degeneracy pressure. However, there are still a significant number of protons in a white dwarf. Under those high densities, do the protons form a degenerate gas?

In addition, are white dwarfs supported by proton degeneracy pressure as much as they are by electron degeneracy pressure?


texts: Current estimates for the number of stars in the universe are about 10^22. However, that number has changed several times as new observations have come forth. How has the estimate of the number of stars in the universe changed over time? What was the estimate after Hubble's findings? What was the estimate in the 70s, 90s, etc., and what new discoveries forced those estimates to change?


texts: How can we tell how many exoplanets a star has?

texts:
Part of this answer (discussing the moving center of mass of our solar system) explains that this movement is one way we know a star has planets:

  Bonus: We use this phenomenon to find planets outside the Solar System! If a distant star is observed to 'wobble' or oscillate about it's mean position, we can use that data to infer the presence of one or more exoplanets, and calculate their mass.

It makes sense to me that we could tell that a start has some planet(s) in orbit around it, based on how they affect the star's position. But is this movement really precise and predictable enough to pin down how many exoplanets, and how large they are? Can we produce a "map" of a distant solar system based on this movement?

If needed, we can even restrict the discussion to planets that are "big enough" to detect. For example, when performing this exercise on our Sun, would we be able to accurately predict at least the four gas giants, even if we couldn't get Mercury?


texts: Is the shape of observable universe and shape of space same?

texts: I have read that Euclidean space is considered as flat space and Minkowski space is flat space-time. So when we say our observable universe is flat are we saying that the space is flat or space-time is flat? I know the Planck data has proved the space to be flat but is it only space or space-time? Since in general relativity matter curves space-time and not just space.


texts: So, what exactly is an 'ultra-cool' dwarf star?

texts:
The TRAPPIST-1 system is around an ultra-cool dwarf star. I went looking for more information on that kind of star, and found very little. The Wikipedia article on it lengthened from a minimal stub to one paragraph in the 15 minutes from when I began the search, after the press conference on the discovery of 7 Earth-sized planets in orbit around TRAPPIST-1.

What kind of star is this?


texts: Closest stable orbit about the sun of an asteroid?

texts: What is the closest perihelion an asteroid in stable orbit could have? Is there a limit defined by orbital mechanics or could an object be in stable orbit, only too close that melting or sublimation would prevent the object from surviving?


texts: Is there matter forming in the inflationary space?

texts:
Following the answers and the line of thinking from here Is there enough hydrogen left after a star dies so another star will have enough to light up?

I was wondering if is there any possibility to know if, where new space is created between the galaxies, is there also created any kind of new matter ( i.e. hydrogen, or subatomic particles that could eventually form atoms, which will lead to forming new stars and galaxies ) ?


texts: Searches for dipole form of anisotropies (on luminosity distances) in the universe with type Ia supernovae

texts:
There are many articles like this one Testing the isotropy of the Universe by using the JLA compilation of type-Ia supernovae (PDF, arxiv.org) trying to search for a dipole effect in cosmology with type Ia supernovae (used as standard candles). The idea of this kind of search is to do a test of the cosmological principle on a phenomenological approach independantly of some given models (e.g. Tolman-Bondi anistropic model).

They consider a dipole effect on the distance modulus $ \mu $: $$ \mu \leftarrow \mu \times \left(1+A_D (\hat{\textbf{n}}\cdot \hat{\textbf{p}})\right)$$ where $\hat{\textbf{n}}$ is unitary vector pointing in the direction $(l,b)$ of the dipole with an amplitude of $A_D$ and $\hat{\textbf{p}}$ points the direction of each Type Ia supernova.
The distance modulus is related to luminosity distance $d_l$ as $ \mu =5 \log \left(\frac{d_l}{10 pc}\right)$.

There is something that bothers me a bit:

Our peculiar velocity to the universe is usually estimated with the CMB dipole measuments, right? And so the reshifts of SNIa are computed in the CMB frame using this correction.

So, supposing we have that kind of cosmological ansisotropy, how it is not mistakenly corrected with the dipole anisotropy of the CMB (and could also be why all searches of this kind give us an amplitude of anisotropy compatible to 0)?

I mean, the CMB dipole implies a velocity of $369.5\pm3.0$ km/s in the direction of $l=264.4^{\circ}\pm0.3^{\circ}$ and $b= 48.4^{\circ}\pm0,5^{\circ}$ according to COBE measurments. But what if the CMB dipole is not just due to our peculiar velocity but also to an unknown "cosmological effect"? This effect would be corrected and nothing will be see when try to make a dipole fit on SNIa measurement, right?


texts: How do I plot orbit planets and satellites?

texts:
I need to find the way to plot orbit planets and satellites in a coherent way. Let's say I want to generate a chart of the Moon's orbit around the Earth, or the jovian satellites around Jupiter.
The hard way would be the use of a CAD software, manually. However, I'm not sure if I will be able to represent the orbits excentricity. And it would be a slow process, one by one...
My idea is to do it programmatically and recursively, so I can generate hundreds of charts for my articles. Can AstroPy do it? I'm using Debian 9 and I checked poliastro, but I had problems with the requirements... I also checked Xephem, and I could not compile it (it's too old, I think). Could Celestia or Kstars do that?
Any Linux/Unix tool would be fine! Especially if it works from the command line.
EDIT: I tried to compile ORSA without luck. Also failed.
I'm discovering the combo of python3-ephem with gnuplot. I can generate the data with Ephem and plot it with Gnuplot. What do you think about this combo?
EDIT 2:
Not only python3-ephem... Skyfield and REBOUND are good options!


texts: Why don't Australia/Russia have large optical telescopes?

texts:
Australia and Russia have large land areas. However, I am only aware of one 3.9m telescope in Australia. As far as I know, there are lots of mountains in Australia and Russia.

So why are there not many large optical telescopes?

The reasons for each country may differ because of their latitudes.


texts: Visual binary star with circular apparent orbit

texts: Does anyone know a good example of visual (or interferometric) binary star apparent orbit of which looks circular due to projection though in fact is markedly elliptical? I need that for teaching.


texts: Are there any cases in which the gravitational lensing of a quasar has been observed in both visible light and in radio waves, and if so, is the radio-wave resolution good enough to show that the lensing at these two different wavelengths is the same?


texts: Client-side JavaScript astronomy libraries which are not based on node.js?

texts:
Are there any client-side JavaScript astronomy libraries? Almost every library I see appears to be server side for node.js. I realize some node.js libraries can be adapted for client-side use, so I'd also be happy with a server-side library that happens to work client-side as well.
EDIT: I did find https://github.com/mourner/suncalc but it appears to be abandoned, has 35+ open issues, many related to accuracy, and, despite its description, appears to give only solar and lunar positions, not planetary positions.


texts: Beginner looking for extra eyepieces for telescope

texts: I have a Bushnell Voyager Reflector telescope. It came with 2 eyepieces: k4mm and k20mm. And I am trying to find additional eyepieces to zoom in further on planets and stars, but the bushnell website doesnt offer any. I am unsure if other brands will work with the telescope. Or is my teleacope not powerful enough to look further out.


texts: How much does cloud cover affect viewing the total eclipse

texts:
As I look for a spot for viewing the total eclipse, how much will cloud cover affect the viewing experience? Is there a threshold list like:

Under x % won't have a noticeable difference
Over x % you won't be able to see the stars
Over x % you won't be able to see the sun's atmosphere
Over x % you will only notice the darkening


texts:
In this diagram of the regions and approximate scale of an AGN:

which is from this paper, what do the x and y scales log(z/pc) and log(r/pc) mean? I'm guessing z is the z-direction ('upwards') and r is radius, but what is the meaning of the pc bit?


texts: At the center of massive galaxies there is a huge and massive black hole that radiates gamma rays. So at the center of the Milky Way there is also a huge and massive black hole and it also radiates gamma rays. Do these gamma rays reach our solar system? What is the age of black hole in the center of our galaxy? Does it still radiates gamma rays?


texts: Is the total energy of the CMBR diminishing with time? Since it's existence the volume of the Universe (at least the visible part) has grown by a factor three, which is to say, the number of photons per unit volume is diminished by a factor 27. But the wavelength of these photons has grown by a much bigger factor. So you should think the total energy of all the CMBR photons has become less over time.


texts: How was astronomical data meant to be handled on HST precursors?

texts:
The first drafts for a large space telescope such as Hubble were made in the 60's, and the idea of a space observatory originated long before that. From Wikipedia:

  In 1968, NASA developed firm plans for a space-based reflecting telescope with a mirror 3 m (9.8 ft) in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope (LST)

Back then, CCDs weren't a thing yet (W. S. Boyle; G. E. Smith (April 1970). "Charge Coupled Semiconductor Devices". Bell Syst. Tech. J. 49 (4): 587–593.), and observational astronomy relied on photographic plates. How was the data meant to be collected and sent back to Earth in these projects?


texts:
I want to calculate the Luminosity of a star e.g. Vega but can be any star and came across the formula for Luminosity at the University of Oregon (http://pages.uoregon.edu/soper/Light/luminosity.html).

I'm not an expert on Maths, could anyone help me with the figures to put into the formula.

$$L = (4\pi d^2)b$$

$d$ is the distance but the page doesn't say Parsecs or Light Years.

$b$ is the apparent brightness.

I'm thinking

$$L = ((4\times3.141) \times (25.04^2) ) \times 0.026 = 204.81 \mathrm{(light\ years)}$$

$$L = ((4\times3.141) \times (7.68^2)) \times 0.026 = 19.26 \mathrm{(parsecs)}$$

Which gives me 204.81 which is different to Luminosity which is quoted (40.12) on the Wikipedia page.  Is this the right formula for what I'm after.  Please no scientific equations like what you see on the page linked above.  Any calculations would need to be the type you would put into a non-scientific calculator.


texts: As opposed to spiral galaxies, which are rotationally supported, elliptical galaxies are said to be supported by random motions of their stars. I don't really have an intuitive understanding of this. How do random motions prevent a collapse?


texts: How much difference did the supply of "fresh" r-process elements make to the Solar System.?

texts:
In one of the answers to this question @MartinKochanski made the interesting point that the abundance of elements heavier than iron (r-process elements) in the solar system is probably due to a fairly recent and close (at the time) neutron star merger -- a relatively rare event on a galactic scale.  This means the early solar system was enriched with "fresh" r-process elements, as compared to the "stale" r-process elements that would be typical. For instance the early solar system is conjectured to have been rich in uranium and thorium compared to a "typical" system.

So my question is what differences might this have made to the formation and evolution of the solar system? Would it have had any bearing on the origins of life on Earth?


texts:
Viewing the animations of Oumuamua passing through the solar system, it is apparent that the plane of its hyperbola departs from the plane of the ecliptic by a large amount.  The plane of the ecliptic departs from that of the Milky Way Galaxy by a large amount.  Now that there is so much speculation about a potential artificial origin, one might be forgiven for speculating that there might be a coincidence between the Milky Way Galaxy's plane and that of Oumuama.

What is the angle between the planes of Oumuamua's hyperbola and the Milky Way Galaxy?


texts: Is the full moon full everywhere on the planet?

texts:
When the moon is full, fully lit by the sun, will it appear full everywhere on the planet?

E.g. if I where to set the date for the next three full moons for say: Sweden, Bali and Seattle how different would those dates be?


texts: How does the Helioseismic and Magnetic Imager (HMI) compute the Magnetic Activity Index (MAI)?

texts: I am looking for a description about how this number called Magnetic Activity Index (MAI) is calculated. All I know is that the processing pipeline of the instrument HMI calculates it. But I would like to know how this quantity is defined and computed! I have not yet found anything on google about it, not even an article.


texts: Why is the Sun's density less than the inner planets?

texts: The density of the Sun is $1410~\frac{\text{kg}}{\text{m}^{3}}$ and Mercury's is $5430~\frac{\text{kg}}{\text{m}^{3}}$, but shouldn't the Sun be denser? Because when the Solar System was forming, there was a big disk of debris, and depending on the density of the debris it went closer or further from the centre, which then formed the planets, but the Sun is in the centre, and it's less dense than Mercury, why?


texts:
The definition of the angular diameter distance is the ratio of an object's physical transverse size to its angular size. However when I was reading my textbook, Astrophysics in a Nutshell by Dan Maoz pp.220-221, I am having some trouble trying to understand the notion of angular diameter distance to the last scattering surface. The text calculates the angular diameter distance to the last scattering surface $D_A$:

  Consider flat cosmology (k=0) with no cosmological constant. We wish to calculate the angular size on the sky, as it appears today of a region of physical size
  $$D_s=\frac{2ct_{rec}}{\sqrt{3}}=140 kpc$$ from which light was emitted at time $t_{rec}$. Between recombination and the present time, the Universal expansion is matter-dominated, with $R\propto t^{2/3}$ for this model
  $$\frac{R}{R_0}=\left( \frac{t}{t_0} \right)^{2/3} = \frac{1}{1+z}$$
  and hence we can write
  $$D_s=\frac{2ct_0}{\sqrt{3}}(1+z_{rec})^{-3/2}$$
  The angle subtended by the region equals its size, divided by its distance to us at the
  time of emission (since that is when the angle between rays emanating from two
  sides of the region was set).

I'm not sure what does the last line actually mean..Can someone please elaborate more on this? I just simply take the $D_s$ as the "physical transverse size".

  As we are concerned with observed angles, the type
  of distance we are interested in is the distance which, when squared and multiplied
  by 4π, will give the area of the sphere centered on us and passing through the
  said region. If the comoving radial coordinate of the surface of last scattering is r, the required distance is currently just $r\times R_0$ and is called the proper motion distance. The proper motion distance can be solved using null geodesic in the FRW metric
  $$\int_{t_{rec}}^{t_0} \frac{c dt}{R(t)} = \int_{0}^{r}\frac{dr}{\sqrt{1-kr^2}}$$
  Setting k = 0, and substituting
  $$R(t)=R_0 \left( \frac{t}{t_0} \right)^{2/3}$$ and integrate
  $$rR_0=3ct_0[1-(1+z_{rec})^{-1/2}]$$

So I take this as the physical distance of the region from us. The next part is what confuse me:

  However, at the time of emission, the scale factor of the Universe was 1 + z times smaller. The so-called angular diameter distance to the last scattering surface is therefore
  $$D_A=\frac{rR_0}{1+z}=3ct_0[(1+z_{rec})^{-1}-(1+z_{rec})^{-3/2}]$$

How does a physical distance $rR_0$ comes into play in the angular diameter distance, because from its definition it is just $$D_A=\frac{\text{physical transverse size}}{\text{angular size}}$$??


texts: What does this figure imply about the luminosity of pre-main-sequence stars?

texts:
Check out Figure 1 in http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1955PASP...67..154H&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf .

It seems to show that the luminosity of a pre-main-sequence star depends primarily on the mass of the star, and that this basic relation continues to hold, to reasonable accuracy, into the main sequence.  Since there is no fusion in a pre-main-sequence star, is it correct to conclude that the relationship between the mass and luminosity of a pre-main-sequence star does not hinge on fusion physics?


texts:
I've owned my AstroMaster 114EQ for about a year now, and feel like I know my way around pretty well, but I'm only just becoming aware that collimation is recommended. The user manual has instructions on basic collimation. Does this look sufficient? Or should I invest in a celestron 94183 collimation cap, a celestron collimation eyepiece, or something fancy like an Orion LaserMate Deluxe II? The telescope itself cost less than $200 so I can't help but wonder if the fancier collimators are overkill. I've watched this longish video and one must apparently perform some disassembly on the telescope to user the laser collimator. This doesn't seem like the easy 'couple of minutes' adjustment I've seen described.

I'm also confused about the process of adjusting the secondary mirror. If anyone has good instructions about my specific model, I would greatly appreciate it. I'd really like to get it tuned up now that Mars is looking so good.


texts: Are black holes orbiting around each other?

texts:
Obviously, bodies with low gravity force orbit around another one with higher gravity force, if they are close enough to each other.

Is it true also for objects like black holes? Are they even able to be moved by another force?


texts: A question about using both the Simbad and the Gaia archive

texts:
It seems a bunch of people, including myself, have got it into our heads to create a 3D visualisation of the stars around us - one where you get to fly around them rather than view them from the viewpoint of Earth.

This means, obviously, that I'm limited to stars that have parallax values, which in the case of the Gaia archive is about 2 million whereas in Simbad 100 000. The reason why I'd like to use both is that Simbad appears to have more useful information, including names and types of star. Also, amazingly, the highest parallax (in milliarcseconds) in gaia is in the 200s, which suggests that alpha centauri is not in the archive! My hunch is that Simbad is the place to go for nearby and named stars, whereas Gaia has higher numbers.

I imagine (perhaps incorrectly) that there is a way to identify a Simbad star on the Gaia database and vice versa. Gaia has a bunch of id values (solution_id, source_id, random_index) whereas on Simbad there's just the one (Identifier). I have the suspicion that Simbad may have more columns I'm not aware of.

In any case, my plan would be to query both archives, and weed out multiples based on an identifier of some sort. Maybe there's a third database that links the two? Maybe I'm missing something obvious? Perhaps there's the perfect page out there that tells me precisely how to do this which has eluded my googling.

Any help would be greatly appreciated.


texts: Venus' magnitude during inferior conjunction

texts:
I was doing an assignment on Stellarium when I observed Venus to have an apparent magnitude that didn't become less negative than -3, even at inferior conjunctions when we faced the dark side of Venus.

As far as I understand, apparent magnitude is subjective to an observer on earth, and that observer does not see Venus at all during an inferior conjunctions, so the magnitude should rise above zero.

I checked for other inferior and exterior planets and the moon on Stellarium; they all supported my theory.
I thought it was some sort of bug, but apparently it's a scientific fact.
I couldn't find any explanation online.

Did I understand something wrong?


texts:
Since Dark Matter is assumed to interact gravitationally, it follows that the gravitation of massive regular matter bodies should have an effect on it. Even if it is otherwise weakly interacting with regular matter, one might naively expect concentrations of it enter orbit around massive objects, like neutron stars, regular stars, and even (why not) our own planet.

So, a two-fold question:
1) Could there be Dark Matter in orbit around (or within) the Earth?
2) If that were possible, would there be any way for humans to tell if such is actually the case?


texts:
Has the earth always been this size apart from a few occasional additions from time to time.

Meteorites hitting the planet now don’t become part of it and the moon when it collided just spun of into its orbit.

So how, as documentaries show, do chunks of rock stick together to form planets etc?

I get dust particles coming together to make suns eventually but in a vacuum. I don’t get how stuff with a large mass sticks together. When stuff with mass collides today it doesn’t stick together: Kuiper belt, rings of Saturn etc.


texts:
I am looking at this video about the timeline of the future.
It says that the universe will stop expanding in about 1 googol years and become a cold bubble but I am not sure that will be true. There are several theories that can be true, so I am not sure which one. Anyone have some proof that one will be true? Thanks!


texts: What is the oldest known prediction of constellation positions?

texts:
Would someone know of a good reference on the oldest known predictions of constellation positions?

How far ahead could accurate positions be predicted?

For example, could current constellation positions be accurately predicted between A.D. 69 to A.D. 86?


texts: How does the cepheid variables help in finding distance

texts: How did Edwin Hubble find distances using Cepheid variables as 'standard candles'?


texts: Official degrees of earth’s rotation per day

texts: What is the official degree to one decimal point please, of the earth’s rotation in one single day. Can it be confirmed that it is exactly 360.0 degrees using official data? Thank you in advance.


texts: Why do J2000 coordinates change with time in Stellarium?

texts:
I'm new to the field so I'm sure this is a basic misunderstanding.
As I currently understand it, J2000 equatorial coordinates describe the position of objects by referencing a specific point in time (12:00 Jan 1st 2000) to account for the effect of the precessional cycle in the reference point for RA. Presumably this means that the same object measured on different dates should still have the same J2000 coordinates?

In Stellarium, if I look at the RA/Dec (J2000) of a star on one date, then compare to the previous year the J2000 coordinates don't match:

I also notice that if I set the current date/time to 12:00 1/1/2000 the RA/Dec (J2000) and RA/Dec (on date) values are different when I would have expected them to be the same:

Where am I going wrong here?


texts: Understanding Earth Tilt, Sun's Position and Lattitude Calculation

texts:
I've been trying to figure out how to calculate the latitude of my location with just the shadow of a stick cast by the sun. I am doing this under the assumption that I am stranded in some place with no almanac or any other data. (not even day or month).

I want to find the latitude roughly. But I have problems picturing the globe along with it's tilted axis and the angles formed by the shadow.

Could you help me understanding the concept and give me a definitive picture of angles that can be calculated. A perspective like a side-view of the earth and sun (orbiting plane - perpendicular to the screen) would help me understand much better.

Here is my efforts to understand so far -

I am not an expert nor have a good background in maths (my maths teacher ruined my life). So please explain me in simple words.


texts: If a person were standing on the surface of Mars, under ideal viewing conditions for seeing a meteor: no clouds, the sun has long set, would a meteor be visible to the naked eye considering how thin the Martian atmosphere is?


texts: If my eyes were really big, would I see radio waves?

texts: Considering an assertion that the fundamental difference between different ‘types’ of electromagnetic waves is frequency/wavelength, and larger antennas are needed for longer wavelengths, if my eyes were larger proportional to the difference in wavelength between visible light waves and radio waves, would they see radio waves?


texts: What is 'surface acceleration' of a planet or other body? As opposed to its 'g' (gravity)?

texts: The Mars Fact Sheet at NASA (their NSSDC or NSSDCA section, whatever that means) lists a value for 'surface acceleration' just below one for 'surface gravity'.  They also have the values for Earth, for comparison.  The values for surface acceleration are very similar to those for gravity, just slightly smaller...


texts:
I am looking for an appendage to attach to the end of a telescope which splits the image into two eyes instead of one.

Does such an appendage exist?


texts: K-correction vs resizing photometric bands

texts: Is there a difference between a K-correction (see Wiki https://en.wikipedia.org/wiki/K_correction) and simply "resizing" the bands, e.g. the 2-10 keV x-ray band, with the redshift? I've read that K-correction requires the assumption of a spectral shape. Why is resizing strictly speaking not enough?